The intramolecular Diels-Alder reaction shows great promise for use in natural product synthesis owing to the fact that the reaction facilitates the construction of stereochemically complex bi- and poly-cyclic ring systems. Fundamental studies of the intramolecular Diels-Alder reactions performed in the first year of the three year grant period define the stereochemical features of the cyclization of substituted methyl 2, 7, 9-decatrienoates and methyl 2, 8, 10-undecatrienoates. The data obtained show that steric, conformational, and electronic factors are more important than secondary orbital interactions in determining the stereochemical outcome of the reactions. In the second grant period the fundamental studies will be continued and intramolecular Diels-Alder reactions will be applied in syntheses of vitamin D3 derivatives, steroids, and chlorothricolide. These syntheses are in progress.